1. Field of the Invention
The present invention relates to optical projection screens illuminated from the rear.
2. Prior Art
From the earliest days of optical projection, the optical images have also been projected from the rear through a projection screen. The, projection screen started out as being the most simplest, which was merely a thin sheet of cotton cloth. Such rear projection screens have been improved and examples of such screens are contained in the U.S. Pat. Nos. as follows:
3,257,900 PA1 3,578,841 PA1 4,418,986 PA1 4,509,822 PA1 4,548,469 PA1 4,561,720
While each of the above patents describes a rear projection screen which includes some sort of a lens system to increase the brightness, all of the above rear projection screens contain certain disadvantages. In particular, they are complex and expensive to manufacture since they include multi-element lenses, require multiple cutting tools and require precise registration between the various multiple cutting tools. Still further, the image itself produced through such screens also has certain defects. In particular, the horizontal viewing half angle is narrow, in the range of 15.degree. to 35.degree., the color correction is poor and the light through the screen is not evenly distributed and contains hot spots.